The present invention relates to a magnetic recording medium employed with particular advantage as an external magnetic recording medium for recording digital data.
Magnetic recording media are widely employed in recording tapes, video tapes, computer tapes, disks, and the like. Magnetic recording media are becoming denser and the recording wavelengths are becoming shorter each year. There is also an examination underway as to whether to change the recording method from analog to digital.
Magnetic recording media employing a thin metal layer as the recording layer are being examined in response to the demand for higher density. However, with regard to such practical reliabilities as production properties and corrosiveness, so-called particulate magnetic recording media in which a ferromagnetic powder is dispersed in binder and coated on a support are superior. However, particulate media have poorer electromagnetic characteristics than thin metal films due to low fill rates of magnetic material.
Particulate magnetic recording media are widely employed in which a magnetic layer comprised of a ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide, CrO2, ferromagnetic alloy powder, or the likes dispersed in binders is coated on a support.
Numerous methods of improving the electromagnetic characteristics of particulate magnetic recording media have been proposed, such as improving the magnetic characteristics of the ferromagnetic powder and smoothing the surface. However, these are not adequate for achieving higher density. Further, in recent years, there has been a tendency to shorten the recording wavelength in combination with higher densities; the problems of thickness loss during reproduction and self-demagnetization loss, where output drops during recording as the magnetic layer becomes thinner, have become significant. Accordingly, ultrathin-layer particulate magnetic recording media have been proposed.
For example, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 6-2153650 discloses relatively reducing the total thickness of the magnetic tape and the thickness of the backcoating layer. As specific examples of the magnetic tape described in this publication, in one embodiment the total thickness of the magnetic tape is 10 xcexcm and the thickness of the backcoating layer is 0.5 xcexcm, and in another the total thickness is 9.5 xcexcm and the thickness of the backcoating layer is 0.5 xcexcm. To impart antistatic and running stability properties to the backcoating layer in such embodiments, comparatively microgranular carbon black alone is employed in the former embodiment, and two types of carbon black, one being comparatively microgranular carbon black and the other being comparatively coarse carbon black, are employed in the latter embodiment.
Additionally, a magnetic tape has been proposed in which microgranular carbon black of a mean particle diameter ranging from 10 to 80 nm, coarse granular carbon black with a mean particle diameter ranging from 150 to 500 nm, and microgranular calcium carbonate of a mean particle diameter ranging from 10 to 45 nm are incorporated into the backcoating layer to achieve high surface smoothness in the backcoating layer, reduce the coefficient of friction with guide pins, and achieve good running stability (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 2-7223). The further introduction into the backcoating layer of inorganic powder (for example, xcex1-iron oxide) is also described.
Increasing the bulk recording density of particulate magnetic recording media is nearly equivalent to employing thin layers to reduce the total thickness of the tape. In particulate recording media, it is also necessary to employ thin layers in the backcoating layer; the thinner it becomes, the greater the requirements for improved dispersion and improved coating strength. Further, proper surface smoothness with repeat running stability is required. However, dispersion of the granular substances such as carbon black that are employed as the principal starting material of the backcoating layer is extremely difficult. Even when kneading treatment, sandmill dispersion, and the like are combined, there are limits to the improvement in dispersion, coating strength, and coating smoothness that can be achieved. In particulate magnetic recording media, there is also the problem of poor dispersion of granular substances in layers other than the backcoating layer.
Accordingly, the object of the present invention is to provide a magnetic recording medium that can be employed with particular advantage as an external recording medium to record digital data, having layers, particularly a backcoating layer, in which granular substances such as carbon black are well dispersed. In particular, the object of the present invention is to provide a magnetic recording medium, in which, even in a backcoating layer that has been reduced in thickness, the granular substances such as carbon black contained therein are well dispersed, and good strength and smoothness are afforded.
The present inventors expended considerable research effort to achieve a magnetic recording medium having a layer comprising binder and granular substances and with well-dispersed granular substances such as carbon black and with good strength and surface smoothness. As a result, they discovered that incorporating the compound denoted by general formula (I) below improved the dispersion of granular substances within the layer; the present invention was devised on that basis.
That is, the object of the present invention is achieved by a magnetic recording medium having a layer comprising a granular substance and binder, characterized in that said layer comprises the compound denoted by general formula (I) below:
Axe2x80x94Nxe2x95x90Nxe2x80x94Xxe2x80x94Yxe2x80x83xe2x80x83General formula (I)
(in general formula (I), A denotes a compound capable of forming an azo dye with Xxe2x80x94Y, X denotes a group selected from the divalent connecting groups denoted by the structural formulas given below 
and Y denotes a group denoted by general formula (II) below 
(in general formula (II), Z denotes a lower alkylene group, xe2x80x94NR2 denotes a lower alkylamino group or a nitrogen-comprising five-membered or six-membered saturated heterocyclic ring, and a denotes 1 or 2).
Preferred Aspects of the magnetic recording medium of the present invention are given below:
1. A particulate magnetic recording medium having a nonmagnetic support, on one side of which is provided a magnetic layer, and on the other side of which is provided a backcoating layer, with the backcoating layer comprising microgranular carbon black with a mean primary particle diameter ranging from 5 to 30 nm, binder, and the compound denoted by general formula (I) above;
2. A particulate magnetic recording medium having a nonmagnetic support, on one side of which is provided a magnetic layer, and on the other side of which is provided a backcoating layer, with the backcoating layer comprising microgranular carbon black with a mean primary particle diameter ranging from 5 to 30 nm, coarse granular carbon black with a mean primary particle diameter ranging from 40 to 360 nm, binder, and the compound denoted by general formula (I) above;
3. A particulate magnetic recording medium having a nonmagnetic support, on one side of which is provided a magnetic layer, and on the other side of which is provided a backcoating layer, where the backcoating layer is formed by dispersing a mixture of microgranular carbon black with a mean primary particle diameter ranging from 5 to 30 nm, binder, the compound denoted by general formula (I) above, and nitrocellulose that has been wetted with any compound from among the group consisting of aromatic hydrocarbon compounds, ketone compounds, and ether compounds to form a carbon black coating material, adding a curing agent thereto, and coating the mixture.
4. A particulate magnetic recording medium having a nonmagnetic support, on one side of which is provided a magnetic layer, and on the other side of which is provided a backcoating layer, characterized in that the backcoating layer comprises microgranular carbon black with a mean primary particle diameter ranging from 5 to 30 nm and an inorganic powder with a mean particle diameter ranging from 10 to 250 nm and a Mohs"" hardness ranging from 5 to 9, and in that the surface roughness Ra thereof ranges from 2.0 to 15 nm.
5. A magnetic recording medium in which the inorganic powder with a Mohs"" hardness ranging from 5 to 9 of Aspect 4 is xcex1-iron oxide or xcex1-alumina.
6. A magnetic recording tape having a nonmagnetic support, on one side of which is provided a magnetic layer and on the other side of which is provided a backcoating layer, where the thickness of the backcoating layer ranges from 0.2 to 8.0 xcexcm, the total thickness of the tape ranges from 3 to 10 xcexcm, and the surface roughness Ra of the backcoating layer ranges from 2.0 to 15 nm.
7. A magnetic recording medium having a nonmagnetic support, on one side of which is provided a magnetic layer and on the other side of which is provided a backcoating layer, where an essentially nonmagnetic lower layer and a magnetic layer comprised of a ferromagnetic micropowder dispersed in binder are provided in this order on the nonmagnetic support, the coercivity of the magnetic layer is equal to or higher than 1.43xc3x97105 A/m (1,800 Oe), the product of the saturation magnetic flux density and the magnetic layer thickness of the magnetic layer ranges from 5 to 300 (mTxc2x7xcexcm), and the surface roughness of the magnetic layer ranges from 1.0 to 3.0 nm as the center surface average surface roughness as measured by an optical interference roughness meter.
8. A magnetic recording medium where, in Aspects 1 to 4 above, the mean primary particle diameter of the microgranular carbon black ranges from 5 to 30 nm, the specific surface area ranges from 60 to 800 m2/g, the DBP oil absorption capacity ranges from 50 to 130 mL/100 g, the pH ranges from 2 to 11, and the volatile content is equal to or less than 15 weight percent.
9. A magnetic recording medium wherein, in Aspect 2 above, the mean primary particle diameter of the coarse granular carbon black ranges from 40 to 360 nm, the specific surface area ranges from 5 to 70 m2/g, the DBP oil absorption capacity ranges from 20 to 100 mL/100 g, and the pH ranges from 5 to 11.
The magnetic recording medium of the present invention is described in greater detail below.
The magnetic recording medium of the present invention comprises the compound denoted by general formula (I) below.
Axe2x80x94Nxe2x95x90Nxe2x80x94Xxe2x80x94Yxe2x80x83xe2x80x83General formula (I)
In general formula (I) above, A denotes a compound capable of forming an azo dye with Xxe2x80x94Y. Said A may be any compound capable of coupling with a diazonium compound to form an azo dye.
Specific examples of said A are given below, but the present invention is not limited in any way thereto. 
In general formula (I) above, X denotes a group selected from among divalent connecting groups denoted by the following structural formulas: 
In general formula (I) above, Y denotes a group denoted by general formula (II): 
In general formula (II), Z denotes a lower alkylene group. Z may be represented by xe2x80x94(CH2)bxe2x80x94, wherein b denotes an integer ranging from 1 to 5, preferably 2 or 3.
In general formula (II), xe2x80x94NR2 denotes a lower alkylamino group or a nitrogen-comprising five-membered or six-membered heterocyclic ring. When xe2x80x94NR2 denotes a lower alkylamino group, it may be represented by xe2x80x94N(CnH2n+1)2, where n denotes an integer ranging from 1 to 4, preferably 1 or 2. Additionally, when xe2x80x94NR2 denotes a nitrogen-comprising five-membered or six-membered heterocyclic ring, the heterocyclic ring is preferably represented by one of the following structural formulas: 
In general formula (II) above, Z and xe2x80x94NR2 are optionally substituted with a lower alkyl group or alkoxyl group.
In general formula (II) above, a denotes 1 or 2, preferably 2.
Specific examples of the compound denoted by general formula (I) above are given below, but the present invention is not limited in any way thereto. 
Synthesis examples of the compound denoted by general formula (I) above are given below.